In control theory and praxis, a control system of a vehicle can be viewed upon as a system performing two subsequent separate steps;
a first step comprising calculating a control demand based on operator commands and feedback signals and taking into consideration requirements on stability and performance; and
a second step comprising calculating and distributing actuator signals to the available actuators, based on the control demand, such that said control demand will be realised. A more detailed explanation can be found in e.g., Härkegård, O; Backstepping and Control Allocation with Applications to Flight Control. Linköping University 2003. pp 105-107.
FIG. 1 shows a block diagram illustrating signal pathways in a prior art flight control system design based on such a division into two separate steps. When developing a such control system process feedback couplings may be constructed using any suitable design method such as e.g. PID (Proportional-Integrating-Derivating Control), linear quadratic minimization (LQ), exact linearization (NDI), adaptive methods etc.). The problem of distribution does not have to be considered, when designing software/hardware realising such a first step.
The design of software/hardware realising the second step can then be performed without having to consider the actual control laws used. In the second step, the distribution of the control demand is performed by distributing to the available actuators control signals that, when having had their effect on said available actuators, fulfils said control demand on the vehicle. This can be achieved by a variety of available methods. Either a fixed (non varying) function, in the linear case a matrix, can be used. In this case the distribution will not be able to adapt to varying effectiveness of the different actuators throughout the envelop of the controlled system. Methods are also available which adapt to the effectiveness of the actuators (control surfaces in aircraft case) and also redistributes the required control if any of the actuators should fail or reach its limitation in position or velocity. These methods usually optimize the actuator performance using the given effectiveness of each actuator, under the given constraints, using some norm (2norm, 1norm or infinity norm).
Further in the system of FIG. 1, a control allocation unit 115 takes said control demand and allocates and distributes actuator signal to available actuators of a vehicle 120. During this allocation and distribution, the control allocation unit 115 considers each actuator's ability to create a certain type of control effect, and modifies the distribution accordingly.
Such an allocation and distribution makes it possible to handle, independent of system total performance, varying actuator performance, e.g. rudder efficiency, but also actuator position and speed limitations, and also faults in actuators. Such faults may include loss, i.e., control surface moves freely, and locking i.e., control surface gets stuck in an arbitrary position.
Methods for control allocation and distribution of a control demand, are disclosed in e.g doctoral dissertation “Backstepping and Control Allocation with Applications to Flight Control” pages 105 to 186 by Ola Härkegård, mentioned above.
Prior art control allocation systems have a tendency to build up an undesired phase loss, when said control allocation systems together with available actuators no longer are able to fulfil current control demand. Non-considered phase loss in a regulatory system is highly undesired. With an operator in the control loop, the operator may experience a feeling of being disconnected from the system. Using aircraft design vocabulary this is called “Pilot In the Loop Oscillations” and they are feared by both pilots and designers.
WO 99/09461 to Buck discloses a method and an apparatus for phase compensation in a vehicle control system. The document describes how a single control surface actuation rate limiter is combined with phase advancing technology to handle phase loss during actuation rate limiting. However, this solution takes care of one actuator at a time. Thus, there is a need for a method and an apparatus for performing phase compensation in a system with multiple actuators, where control requirements can be redistributed within a group of actuators.
U.S. Pat. No. 5,528,119 to Rundqwist et al, discloses a method and a device for executing phase compensation in a motor-driven vehicle without control allocation.